Internet lore and science fiction tales suggest that dropping a nuclear weapon on an erupting volcano would halt the eruption. But would that really be the case?

Would it just make the eruption worse by opening up more magma tubes? Would it alter the lava flows? Would it send reverberations through the earth's core and crack the world in half (as suggested by the 1965 classic, Crack in the World)? Since no country is likely to drop a nuclear bomb on a volcano in the near future (well, at least Italy and the U.S. didn't take that train of advice with Mount Etna), let's take a look at the possibilities for what might really happen, to the best of our scientific knowledge.

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Where do we put the nuclear device?

I'll present two options for the placement of the nuclear device – 1) the base of the volcano, causing an explosion to give the magma another place to go, and 2) the mouth of the volcano, caving in or sealing off the opening of the volcano, or in a more extreme case, turning the volcano from a pimple on the crust of the earth to a crater. Also, we'll not specify which type of nuclear weapon being used, as an A, H, or neutron bomb should all have the same general characteristics (extreme heat and an air burst) in the early stages, which are what we'll be concerned with. We'll let later generations deal with the radioactive fallout.

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Magma vs. Nuclear Fission – What Wins?

The temperature at the core of the nuclear detonation could be as much as several million degrees Celsius. This temperature would fall off with distance, but would still be over 6000 degrees Celsius dozens of meters away. This temperature would decrease quickly, and be followed by a shock wave. Magma (and once it is exposed to the air, lava), on the other hand, is typically 600 – 1300 degrees Celsius and mainly composed of silicon dioxide, with this also being the approximate temperature at the mouth of a volcano. Silicon dioxide boils at around 4000 degrees Celsius. The nuclear blast should provide a sufficient temperature to boil the magma, vaporizing it along with enough heat to cause a phase transition of the surrounding rock to vaporize it as well.

Logistics

There is a logistical issue with the nuclear detonation, however. One would have to ensure that the fission reaction within the nuclear warhead occurred prior to it coming in contact with the lava, which would likely melt the warhead and the radioactive material inside and prevent a full detonation. Also, a detonation that happens too early and too high above the volcano would only result in an air blast rushing onto the volcano. This delicate timing situation would call for remote detonation, as the surface tension of the lava will doubtfully be considerable enough to initiate detonation.

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What happens after the initial explosion?

So, we probably have enough heat to vaporize a fair amount of the magma and rock making up the volcano. If the nuclear device is placed at the base of the volcano, the first of the two possibilities we'll look at, the heat accompanying the blast and the following air burst would likely bore a hole into the side of the volcano, if only for a moment prior to a cave in, alleviating pressure on the magma chamber. This would allow more magma to flow out afterward. With careful placement, this would prevent an "unplanned" eruption and at the same time causing an eruption in an alternate direction that would be somewhat controlled.

If the nuclear device is detonated at the opening of the erupting volcano, the lava and a portion of the protruding side wall would likely be vaporized. This is where the variables come into play. Previous test explosions, like the 1962 Sedan nuclear test (which aimed to use nuclear weapons in large scale earth moving), formed a crater approximately 100 meters deep and 400 meters wide (an image of the crater can be seen above). If the detonation occurred with enough force, it could possibly extend its force down to the magma chamber and "plug" the chamber, halting eruption. However, taking into account the amount of material moved in the Sedan test and comparing it to the height of a volcano (often several thousand meters high), this would seem highly unlikely. A blast at the mouth of the volcano would likely just crush part of the top of the volcano. Even if the blast was strong enough the pressure in the magma chamber would have been alleviated temporarily, but I would think it would be likely return over time, causing another volcanic event. The evil magma just has to get out.
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